1. Field of the Invention
This invention relates to starting electric motors, and more particularly to an overhang type starting electric motor for starting the engine of a vehicle.
2. Prior Art
An over-hang type starting electric motor for starting the engine of a vehicle is well known in the art, as disclosed for instance by Japanese Utility Model Application Publication No. 6679/1986.
The specific feature of the structure of an overhang type starting electric motor is, in general, as follows: As is apparent from FIGS. 1 and 2 showing the over-hang type starting electric motor disclosed by the aforementioned Japanese Utility Model Application Publication No. 6679/1986, a front frame 1 has an opening 2, through which an extension 3 of an armature rotary shaft of a DC motor (not shown) provided inside the frame is extended outwardly, and a movable pinion cylinder 4 is fitted through bearings 5 on the extension 3 in such a manner that the pinion cylinder 4 is movable axially and is rotatable around the extension 3. The pinion cylinder 4 is rotatably and slidably supported by a bearing 6 arranged inside the frame 1 and near the opening 2, thus supporting the extension 3 of the armature rotary shaft (hereinafter referred to as "a shaft extension 3", when applicable).
In the starting electric motor, the outer wall of the front end portion (on the side of the opening in the frame) of the pinion cylinder 4 is formed into a pinion 4a, and the rear end portion is formed into a clutch inner 7a which is a part of an overrunning clutch device 7. The outer wall of the middle part of the movable pinion cylinder 4, which is located between the pinion 4a and the clutch inner 7a is a supporting surface 4b in slide contact with the bearing 6 so that the movable pinion cylinder 4 is supported while sliding on the bearing 6.
In FIGS. 1 and 2, reference character 7b designates a clutch outer of the overrunning clutch device 7; 7c, rollers for transmitting torque from the clutch outer 7b to the clutch inner 7a; 8, an oil seal; 9, a dust protective cap detachably mounted on the front end of the movable pinion cylinder 4; 10, a stopper secured to the end of the shaft extension 3 to prevent the pinion cylinder 4 from coming off the shaft extension 3; and 11, a ring gear of the engine.
The operation of the starting electric motor thus constructed will be briefly described.
When the overrunning clutch device 7 is moved in the forward direction (or in the direction of the arrow 12) by a shift lever (not shown), the movable pinion cylinder 4 is slide on the shaft extension 3, as a result of which, as shown in FIG. 2, the pinion 4a comes outside the frame 1 through the opening 2 to engage with the ring gear 11. A power switch for the DC motor is turned on immediately before the pinion 4a engage with the ring gear 11. As a result, torque is transmitted from the armature rotary shaft through the clutch outer 7b of the overrunning clutch device 7 and the rollers 7c to the clutch inner 7a, so that the pinion cylinder 4 is rotated. The rotation of the pinion cylinder 4 is transmitted through the pinion 4a and the ring gear 11 to the engine to start the latter.
After the engine is started in this way, the movable pinion cylinder 4 may be rotated at high speed by the engine before returning to the original position. In this case, the clutch inner 7a of the overrunning clutch device 7 is rotated at higher speed than the clutch outer 7b. Therefore, the function of transmitting torque in one way of the overrunning clutch device 7 is exercised so that the rollers 7c are disengaged from both of the clutch inner 7a and the clutch outer 7b. As a result, the high speed rotation of the pinion cylinder 4 will not transmitted to the DC motor.
On the other hand, in the conventional starting electric motor, both end portions of the movable pinion cylinder 4 are used as the clutch inner 7a and the pinion 4a, and the middle portion is employed as the slidably supporting surface 4b, and therefore the pinion cylinder 4 is relatively long. In order to allow the long pinion cylinder 4 to stably slide on the shaft extension 3 and to sufficiently support the shaft extension 3, two bearings 5 are positioned on the inner cylindrical wall of the pinion cylinder 4 near both ends.
Recently, in order to increase the torque of the ring gear 11, it is intended to make the pinion 4a smaller in the art. If the tooth form of the pinion 4a is made smaller or the number of teeth is reduced, then the thickness m of the teeth base of the pinion 4a is reduced to less than the necessary value, because the bearing 5 is disposed on the inner cylindrical wall of the pinion 4a. Therefore, the pinion 4a may be damaged.
Furthermore, in the conventional starting electric motor, the grease applied for the bearings provided on the inner cylindrical wall of the movable pinion cylinder 4 is removed by the reciprocation of the movable pinion cylinder, and the service lives of the bearings are shortened as much.
Further more, in the conventional starting electric motor, the cap 9 is put on the front end of the movable pinion cylinder 4 as shown in FIG. 1 to prevent the entrance of water or dust, along the cylindrical surface of the shaft extension 3, into the movable pinion cylinder 4. However, the cap 9 secured to the end of the movable pinion may be damaged by the centrifugal force, or it may be damaged or broken being struck against the ring gear 11 of the engine when the starting electric motor is engaged with or disengaged from the engine. On the other hand, the length of the cap 9 is such that the cap 9 surrounds the front end portion of the shaft extension 3 when the movable pinion cylinder 4 is at the rearmost position as shown in FIG. 1. Therefore, when the movable pinion cylinder 4 is moved to the foremost position as shown in FIG. 2, the total length of the starting electric motor becomes considerably long, which lowers the efficiency of installation of the starting electric motor on the engine.
On the other hand, since one of the bearings 5 is positioned in the pinion cylinder 4 at the front end, the distance L between the bearing 5 and the cut-up part 3a of the helical spline of the shaft extension 3 is remarkably long when the pinion cylinder 4 slides. Therefore, a great bending moment is applied to the cut-up part 3a, so that it may be broken. Furthermore, because the front bearing is positioned on the inner cylindrical surface of the pinion 4a which is the front end portion of the movable pinion cylinder 4, the thickness of the teeth base of the pinion is limited, and accordingly the tooth form of the pinion or the number of teeth is limited.